This invention relates to a pulse train presence detector, as can be particularly utilized in a braking system for a transit vehicle having a microprocessor controlling the braking operation. Such microprocessor-control arrangements have found wide-spread acceptance in many fields today, including the field of transit vehicle braking systems which, because of the need to shield the riding public from harm, requires the highest degree of safety considerations. It is a required condition of such braking systems that, in the event of a failure of the microprocessor-control arrangement, in either the hardware or software segments, the braking units are activated to effect stoppage of the vehicle. In some known applications of microprocessor-control systems, particularly in the field of transit or rail-type vehicles, one way of checking the safe operation of the microprocessor is to use the clock pulse, which provides the necessary timing to the microprocessor, to maintain a power-up circuit, the absence of which shuts the microprocessor down. The problem with such an operation is that the clock pulse is not an accurate indication of the working condition of the hardware and software segments of the microprocessor, and, as such, an accuracy problem arises. Additionally, merely shutting down the microprocessor does not always result in the shutdown to a more restrictive or desirable condition of the system it is controlling, namely, a braking system in this situation. Still other microprocessor applications in this area have used multiple, parallel microprocessors with voting arrangements to verify the integrity of the microprocessor control arrangement. Inherently disadvantageous in this approach, is the increased cost due to the increased number of components required.